This invention pertains to the electronics art and, more particularly, to a high performance summing amplifier.
Summing amplifiers, are well known, particularly in the communication art. The function of a summer is to combine two or more signals thereby producing an output mixed signal.
FIG. 1 is a schematic diagram of a typical summer amplifier topology. Here, a plurality of signal sources S1-S3 are to be linearly summed in a proportion determined by input series resistors R.sub.1 -R.sub.3, respectively. These signals are fed through summing resistors R.sub.1 -R.sub.3 to the inverting input 12 of a high gain amplifier 10. The non-inverting input 14 of amplifier 10 is connected to ground potential. A feedback resistor R.sub.F couples from the output 16 of amplifier 10 to the inverting input 12. Ideally, the signal S.sub.out appearing at the output 16 of the summing amplifier is of the form: ##EQU1##
Also, under ideal conditions, inverting input 12 of amplifier 10 would be at virtual ground potential, whereby each of the signals S.sub.1 -S.sub.3 would be isolated from all other signals, thereby eliminating the possibility of cross-talk or feed through.
Unfortunately, the summing amplifier designs known in the prior art have not realized ideal performance, particularly at high frequencies. Thus, the impedance at the inverting input of such prior art devices has not been maintainable at virtual ground thereby resulting a degraded isolation characteristic. In many applications, such as the transmission of multiple channels of information over a single channel, this isolation degradation cannot be tolerated.